The field of atomic emission spectroscopy was developed at the time of Johann Balmer, Johannes Rydberg, Friedrich Paschen, and their contemporaries, during the late 1800s, when flame was used as an excitation source. Atomic emission spectroscopy is now applied to almost every scientific discipline including physics, astronomy, chemistry, biochemistry, medicine, forestry and agriculture, geology and mining, forensic science, environmental protection, metallurgy and metal production, etc. Innovations in this area are, therefore, significant and valuable.
Several methods are presently used to measure the optical emission spectra of solid, liquid and gas samples. Precision methods use a thermal (flame), electrical (arc/spark) or optical (laser) energy source to 1) remove debris from the sample surface, 2) heat a portion of the sample sufficiently to dissociate atoms in a gaseous vapor, and 3) excite the electron structure of the vapor atoms to temperatures sufficient to generate optical emission. A spectrometer records the resulting optical emission signatures.
Quantitative analysis emerged about 1930 with the use of alternating current (AC) arcs, interrupted direct current (DC) and AC arcs, as well as high voltage sparks. Sparks delivered sufficient energy to ionize sample vapors and reveal a wider array of emission lines. Electrical discharge (ED) techniques are widely used in some of the most precise analytical instruments built today, but application of ED techniques is limited to samples that are conductive. Such sources can deliver large energy to samples at low cost, and the energy delivery can be controlled to achieve high quality spectra. On the other hand, the precise location of plasma formation by ED techniques is difficult to control, making it, in turn, difficult to achieve high stability of the sensed optical signals. Indeed, erratic motion of the plasma location across the sample surface during the course of a measurement adds noise to the sensed optical signal. Moreover, the geometrical requirements, both with regard to sample proximity to the electrode and axial centering, limit the solid angle from which spectral signal may be obtained.
Within a decade of their advent, lasers were used as excitation sources for optical emission spectroscopy (OES), creating the field of Laser Induced Breakdown Spectroscopy (LIBS). This technique allows for fine spatial sampling of materials, since laser beams may be tightly focused and precisely directed. Moreover, LIBS imposes fewer geometrical restrictions on the signal acceptance angle, and, additionally, makes possible the practical measurement of nonconducting samples, including liquids and gases. LIBS equipment is expensive and far more complex than electrical discharge systems. At this time, the most precise results are obtained when multiple laser pulses are used, timed in rapid succession, and when “pre-ablation” laser pulses are delivered in the air above the sample prior to spectral measurement. Multiple laser pulses, delivered in rapid succession may require the use of multiple lasers, dramatically increasing the cost and complexity of such systems.
Rasberry et al., Laser Probe Excitation In Spectrochemical Analysis, Appl. Opt., vol. 6, pp. 81-86 (1967), incorporated herein by reference, reported the use of a laser providing energetic pulses of greater than 100 mJ per pulse to ablate significant quantities (e.g. 1.0 μg) of analyte and then to subsequently initiate electrical discharge in the resulting material-rich laser-induced plasma. The choice of excitation source was driven by its demonstrated ability to operate as a stand-alone LIBS source. In fact, the researchers routinely compared the spectra of laser-only (i.e. LIBS) optical emission with the spectra generated by laser-initiated electrical discharge (i.e. using electrodes to further excite the emissions).
All laser-excited OES practiced to date, whether in conjunction with electrical discharge or laser self-sustained, employs laser pulses of sufficient energy per pulse to ablate a volume of material from the surface of the sample that is adequate for spectroscopic observation. The term “ablation energy threshold,” as used herein, and in any appended claims, refers to the instrument-specific minimum energy, per unit area of the sample, that is required in order to ablate atoms from the surface of the sample in sufficient number (typically greater than 1 microgram) and at sufficient quantum excitation to provide a signal-to-noise of at least unity in an atomic emission line in a single pulse. The ablation energy threshold depends on the material composition of the sample, on the spectral composition of the exciting pulse, and also, it has been shown, upon the pulse power and duration (and not merely upon their product).
The requirement, in all practice of LIBS to date, that the energy per pulse exceed the ablation energy threshold, imposes a lower limit on the size of laser that is required. Typically, energies per pulse in excess of 20 mJ are employed, and often substantially greater energy per pulse. A specific example of a laser routinely used for LIBS is the Big Sky Laser Technologies' Ultra Nd:YAG 50 mJ laser. The optical head size is 50 mm×75 mm×200 mm, and its controller/cooler unit measures 380 mm×380 mm×190 mm and dissipates>200W. This laser is water-cooled, requires regular periodic maintenance and will perform a maximum of 20 measurements per second.
The size and weight of lasers that are required by LIBS as currently practiced preclude hand-held practice of LIBS spectroscopy.